Tubular structures and coatings have been prepared by a number of techniques, each of which has limitations for each application. For biomedical applications, a limitation is the abundant material required to prepare structures of limited size and shape, which can prove costly. For porous polymeric tubes, also known as hollow fiber membranes (HFMs), tubes with wall thicknesses on the order of hundreds of microns are prepared. There is no suitable method to prepare concentric, long HFMs, with thin walls, whether by dip-coating, spinning, or centrifugal casting, among others. As will be described in more detail, the invention comprises a process to prepare HFMs, coatings or any hollow structure, with a broad range of wall and surface morphologies, dimensions and shapes. Such wall morphologies allow HFMs to be manufactured with considerably different transport properties while maintaining similar mechanical properties.
HFMs are commonly prepared by phase inversion through an annular die (or spinneret) where the solvent/non-solvent system controls many of the resulting properties, such as morphology of the wall structure. The dimensions are controlled by the spinneret, which must be finely tuned for concentricity. While the spinning technique has a proven record commercially, it requires abundant material and requires a certain amount of art to prepare reproducible HFMs.
Centrifugal casting is a process used to make a wide number of structures, both tubular and non-concentric (U.S. Pat. Nos. 5,266,325; 5,292,515). For manufacturing tubular shapes, a cylindrical mold is partially filled with a monomer, polymer melt, or monomer solution, and with air present inside the mold, coats the periphery of the mold under centrifugal action. The material spun to the outer portion of the mold is then held in place using temperature changes (cooling), polymerization or evaporation of the solvent. For this process, two phases are present inside the mold (gas and liquid) before rotation; phase separation is not necessary for tubular formation. Wall morphologies are only attained by the addition of a porogen (salt, ethylene glycol etc.) that is leached out post-polymerization. Since a gas is required in the mold to form a tube (compared to a rod), attaining small diameter tubes with a small inner diameter on the micron scale cannot be achieved. Surface tension between the liquid and the gas inside the mold prevents miniaturization of the inner diameters for tens of centimeter length tubes.
For dip-coating, tubes are formed around a mandrel that is sequentially dipped in a polymer solution and non-solvent system, thereby coating the mandrel with the polymer via a phase inversion process. Alternately, the mandrel may be dipped in a polymer solution and the solvent left to evaporate. By these methods, the uniformity of the tube wall along the length of the tube is not well controlled.
It would therefore be very advantageous to manufacture tubes within a size regime, concentricity and with a multi-layering capability that is not presently achievable with the aforementioned methods. Furthermore, it would be desirable to have composite structures that were manufactured within a size regime, concentricity and multi-layering not presently available with the aforementioned methods. For example, composite structures allow soft tissue moduli to be matched with soft (low moduli) materials, yet to have a design that provides strength and patency, which is important to device utility.
Current coatings technologies have limitations in terms of the uniformity of the coating, thickness of the coating and coating porous materials. For example, dip-coating provides uneven coatings and the coating infiltrates the porous material. Spray-coating achieves a conformal coating that inherently coats the each pore.
It would be desirable to provide a method of producing tubular or non-tubular structures which can be used in a variety of physiological or other applications which can be produced using a wide variety of materials and which can include composites of biological materials.